1. Field of the Invention
The invention relates to an inertial sensor such as a gyroscope, specifically relates to a semiconductor acceleration sensor installed in cars, aircrafts and portable equipments for detecting the acceleration in the three-axes (an X-axis, a Y-axis and a Z-axis), each of which is mutually orthogonal.
2. Description of the Related Art
The conventional semiconductor acceleration sensor is disclosed in the reference JP 2004-294230. The semiconductor acceleration sensor disclosed in the reference JP 2004-294230 is formed with a supporting substrate and an SOI (Silicon On Insulator) substrate, which includes a thin silicon layer formed on a buried oxide layer formed directly on the supporting substrate.
As specifically disclosed in the drawings of the reference JP 2004-294230, the semiconductor acceleration sensor includes a frame, a proof-mass, four flexible members, first locking members, second locking members, first recesses, second recesses and projections. The proof-mass is disposed within the frame by slits, and the proof-mass includes a core member and four collateral proof-masses, each of which is connected to the core member. The four flexible members, which are formed of the thin silicon layers, are disposed cruciately, and each of the flexible members connects the frame to the proof-mass. Each of the first locking members, which is formed of the thin silicon layer, extends from the frame onto the edge of the proof-mass passing over one of the slits. Each of the first recesses, which is rectangular-shaped, is formed at the edge of the collateral proof-mass. The depth of each first recess is the same as the total thickness of one of the first locking members and the buried oxide layer, and each first locking member extends in the first recess. Each of the second locking members, which is formed of the thin silicon layer, extends from the collateral proof-mass onto the edge of the frame passing over one of the slits. Each of the second recesses is formed at the edge of the frame. The depth of each second recess is the same as the total thickness of one of the second locking members and the buried oxide layer, and each second locking member extends in the second recess. Therefore, an overlap area with space between the first locking member and the collateral proof-mass is created at each first recess. As well, an overlap area with space between the second locking member and the frame is created at each second recess. Further, each of the projections, whose thickness is the same as the buried oxide layer, is formed at the edge of the collateral proof-mass. The projections are disposed to sandwich the one of the flexible members.
In the semiconductor acceleration sensor disclosed in the reference JP 2004-294230, each of the first recesses is located at the center of the edge of one of the collateral proof-masses, and each of second locking members is located at the corner of the one of the collateral proof-masses.
According to the semiconductor acceleration sensor disclosed above, since the displacement of the proof-mass in the z-axis direction is restricted, the resistance characteristics against impact shock are improved.
However, since the displacement of the proof-mass is restricted by the first combinations of one of the first locking members and one of the first recesses and the second combinations of one of the second locking members and one of the second recesses, when the excessive external force such as drop impact (generally over 8000 G impact) is applied, the proof-mass is displaced in a direction, which is in parallel to the X-Y plane around the Z-axis. When the tip of the second locking member collides with the side wall of the second recess, stress is focused on the base of the second locking member because the second locking member is rectangularly-shaped, in other words, because the second locking member extends from the proof-mass in the right angle. As a result, the second locking member may be broken so that the flexible member may be transformed due to the breakage. If the flexible member is transformed, it may be impossible to detect the acceleration.